1. Field of the Invention
The present invention relates to a strut-type suspension used in a vehicle such as a car, and a suspension compression coil spring for use in the suspension.
2. Description of the Related Art
Patent Document 1 (Jpn. Pat. Appln. KOKAI Publication No. 2000-103216) or Patent Document 2 (Jpn. Pat. Appln. KOKAI Publication No. 2004-150637) discloses a strut-type suspension which is used in a vehicle such as a car. In these strut-type suspensions, a compression coil spring, a lower spring seat, an upper spring seat, and a shock absorber are provided. The compression coil spring serves as a suspension spring. The lower spring seat is disposed on the lower end side of the compression coil spring. The upper spring seat is disposed on the upper end side of the compression coil spring. The shock absorber passes through the inside of the compression coil spring. The shock absorber includes elements such as a cylinder in which fluid is accommodated, a rod which is inserted into the cylinder to be freely extended and retracted, and a damping force generation mechanism. The damping force generation mechanism has the function of attenuating the extending and retracting movement of the rod.
In order to reduce sliding resistance that is produced between the cylinder and the rod of the shock absorber, in Patent Document 1, the position of a coil central axis of the compression coil spring is offset to the outer side of the vehicle with respect to the shock absorber. In the shock absorber of Patent Document 2, a pitch angle of the compression coil spring is varied in accordance with a position of turns of a wire from an end.
In this field, from a standpoint of reducing the weight of a vehicle, reducing the weight of a compression coil spring for suspension (hereinafter referred to as a suspension compression coil spring) is strongly desired. It is known that in a suspension compression coil spring, a stress which is produced in each part of the wire is generally not constant in a state where a load is applied. In order to reduce the weight of a compression coil spring as described above, it is effective to approximate a stress distribution of a wire to a uniform distribution (i.e., make it flat) as much as possible. As one way of uniforming the stress distribution of a compression coil spring, varying the diameter of a wire in one turn has been proposed, as described in Patent Document 3 (Jpn. Pat. Appln. KOKAI Publication No. 59-219534). In the compression coil spring shown in FIG. 6 of Patent Document 3, for example, an external force acting point is offset in a radial direction of the coil with respect to the coil central axis. Further, a wire diameter on the offset side is reduced.
The inventors of the present invention have carried out intensive study to uniform a stress of a compression coil spring used in the strut-type suspension. As a result, it has been found that when the coil central axis of the compression coil spring is offset to the outer side of the vehicle with respect to the shock absorber, if a wire diameter on the inner side of the vehicle is reduced, variations in the stress distribution of the wire are more significant on the contrary, instead of making the stress distribution more uniform.